Crossed-field plasma devices and other Penning discharge devices are known in the art. In such devices, an ionizable gas in a space between a pair of electrodes is subjected to electric and magnetic fields, at right angles. The magnetic field is generally parallel to the long dimension of the electrodes and the electric field is transverse. When a gas discharge is struck between the electrodes, ions and electrons in the resulting plasma are influenced by the fields, with electrons traveling a path whose direction is generally perpendicular to the plane of the crossed-fields. As a result, electrons generally proceed down the length of the electrodes by following a helical path in the interelectrode space. The combined fields produce an electron motion which allows the electrons to follow a longer effective path and create a resultant greater level of gas ionization.
Such structures have heretofore been used to create plasma guns, e.g. see U.S. Pat. Nos. 3,005,931 to Dandl, 3,201,635 to Carter and 3,238,413 to Thom et al. Additionally, such structures have been employed as parts of ion accelerators, e.g. see U.S. Pat. Nos. 3,155,858 to Lary et al and 3,345,820 to Dryden, and as part of a conduction control device, e.g. U.S. Pat. No. 4,322,661 to Harvey. Furthermore, such a structure has been employed as an electron generator, but with less than satisfactory results, i.e. see "The Characteristics of Electrical Discharges in Magnetic Fields", edited by Guthrie et al., first edition, McGraw-Hill Book Company, 1949, Chapter 10, "Discharge Cathodes" by Parkins, pp. 334-344. Parkins disclosed a crossed-field discharge device wherein electrons exited to their point of use along magnetic field lines. His structure employed a source of gas to feed a continuous discharge, thereby making it difficult to maintain the desired low pressure level within the beam acceleration structure, with the result that high voltage electron beam generation was not possible.
In order to generate high voltage electron beams (1 kilovolt to greater than 1 megavolt), the cathode must be electrically insulated from the anode by an appropriate vacuum space. Depending on the electron beam voltage and current density, a predetermined quality vacuum is required, typically better than 10.sup.-4 mm of Hg. However, to strike a crossed field discharge typically requires of the order of 10.sup.-2 mm Hg gas pressure. Thus a crossed field plasma cathode must generate the electron beam in an area of "high" pressure while at the same time conducting electrons, without hindrance, to an area of lower pressure (e.g. 10.sup.-4 mm Hg), and maintaining the highest level of electron discharge possible.
Accordingly, it is an object of this invention to provide an improved, crossed-field, electron beam generator capable of providing a high voltage electron beam.
It is still another object of this invention to provide an improved, crossed-field electron beam generator which is constructed to maintain an optimum internal discharge gas pressure while, at the same time providing a high voltage beam into a region of lower gas pressure.
It is a further object of this invention to provide a crossed-field electron beam source wherein arcing is avoided.